The present invention relates to an MR (magnetic resonance) imaging method and MRI (magnetic resonance imaging) apparatus, and more particularly to an MR imaging method and MRI apparatus which can reduce residual magnetization caused by gradient pulses.
In prior art, Japanese Patent Application Laid Open No. H10-75940 discloses a technique involving:
(1) a phase shift measurement method comprising the steps of: executing a prescan sequence comprising transmitting an excitation RF pulse, transmitting an inversion RF pulse, applying a phase encoding pulse on a phase gradient axis, applying a read pulse on a read gradient axis and applying a rewinder pulse on the phase gradient axis, and subsequently transmitting an inversion RF pulse, applying a dephaser pulse on the phase gradient axis and collecting data from an echo while applying a read pulse on the phase gradient axis; and measuring a phase shift in subsequent echoes caused by the effect of eddy current or residual magnetization from the phase encoding pulse and so forth based on phase data obtained by performing one-dimensional Fourier transformation on the collected data, and
(2) an MR imaging method employing a fast spin echo pulse sequence that involves repeatedly executing the following steps a plurality of times with a varying phase encoding pulse: after transmitting an excitation RF pulse, transmitting an inversion RF pulse, applying a phase encoding pulse on a phase gradient axis, collecting data from an echo while applying a read pulse on a read gradient axis, and applying a rewinder pulse on the phase gradient axis, thereby collecting data for a plurality of echoes with one excitation RF pulse, wherein a compensation pulse for compensating for a phase shift amount measured by the phase shift measurement method as described regarding (1) is either incorporated in the phase encoding pulse or appended either immediately before or immediately after or immediately before and after the phase encoding pulse, or incorporated in the rewinder pulse or appended either immediately before or immediately after or immediately before and after the rewinder pulse.
The above technique presupposes that the phase shift amount measured by the phase shift measurement method of (1) is equal to the phase shift amount generated in the fast spin echo pulse sequence of (2) before the compensation pulse is added.
However, these amounts are not always equal in the MRI apparatus because of the magnetic hysteresis characteristics of a magnetism conditioning plate or the like, and the presupposition in the above technique does not always hold. This will be explained with reference to FIGS. 1 and 2 hereinbelow.
FIG. 1 is a pulse sequence chart according to a conventional fast spin echo (FSE) technique.
In an FSE sequence SQ, an excitation RF pulse R and a slice selective pulse ss are first applied. Next, a dephasing pulse gx1 is applied on a read gradient axis. Next, a first inversion RF pulse P1 and a slice selective pulse ss are applied. Next, a phase encoding pulse gy1i is applied on a phase gradient axis. Then, data is collected from a first echo echo1 while applying a read pulse gxw. Thereafter, a rewinder pulse gy1ri is applied on the phase gradient axis having the same area as, and a polarity opposite to, the phase encoding pulse gy1i. Reference symbol i represents a repetition number of the FSE sequence SQ in FIG. 1, and i=1xe2x80x94I (for example, I=128).
Next, a second inversion RF pulse P2 and a slice selective pulse ss are applied, a phase encoding pulse gy2i is applied on the phase gradient axis, data is collected from a second echo echo2 while applying a read pulse gxw, and then, a rewinder pulse gy2ri is applied on the phase gradient axis having the same area as, and a polarity opposite to, the phase encoding pulse gy2i.
Thereafter, and similarly, a j-th inversion RF pulse Pj and a slice selective pulse ss are applied, a phase encoding pulse gyji is applied on the phase gradient axis, data is collected from a j-th echo echoj while applying a read pulse gxw, and thereafter, a rewinder pulse gyjri is applied on the phase gradient axis having the same area as, and a polarity opposite to, the phase encoding pulse gyji, repeatedly for j=3xe2x80x94J (although J=8 for example, FIG. 1 shows a case that J=3).
And finally, a killer pulse of a large amplitude is applied on the phase gradient axis.
FIG. 2 is diagram of the magnetic hysteresis characteristics of a ferromagnetic material such as a magnetism conditioning plate in the MRI apparatus.
The magnetization strength B of the ferromagnetic material such as the magnetism conditioning plate varies as indicated by a main loop Ma when the external magnetic field strength H is substantially changed, while it varies as indicated by a minor loop Mi when the change in the external magnetic field strength H is small. A gradient pulse corresponds to the small change in the external magnetic field strength H. Accordingly, application of a gradient pulse causes the magnetic strength B of the ferromagnetic material such as the magnetism conditioning plate to vary as indicated by the minor loop Mi.
Thus, the MRI apparatus has residual magnetization varying depending on a history of applying gradient pulses, owing to the magnetic hysteresis characteristics of a ferromagnetic material such as a magnetism conditioning plate.
However, since the prescan sequence as described regarding (1) does not take care of residual magnetization due to the killer pulse kp, the phase shift amount measured by the phase shift measurement method of (1) is not equal to the phase shift amount generated in the fast spin echo pulse sequence of (2) before the compensation pulse is added. That is, residual magnetization due to a killer pulse in an (ixe2x88x921)th FSE sequence SQ affects all the echoes in an i-th FSE sequence SQ.
Moreover, the prescan sequence of (1) is in the form of a partially cut-out FSE sequence up to the first echo, and the history of applying gradient pulses of the prescan sequence is not equal to that of the MR imaging scan at and after the second echo. Therefore, the residual magnetization affects the second echo and the following echoes.
Thus, the conventional technique as described above has a problem that the effect of residual magnetization caused by gradient pulses cannot be sufficiently reduced.
It is therefore an object of the present invention to provide an MR imaging method and MRI apparatus which can sufficiently reduce the effect of residual magnetization due to gradient pulses.
In accordance with a first aspect of the present invention, there is provided an MR imaging method comprising the steps of applying a gradient pulse having either a positive or negative polarity on a gradient axis, and thereafter applying a residual magnetization reducing pulse having a polarity and amplitude to reduce residual magnetization caused by the gradient pulse.
In the MR imaging method of the first aspect, a residual magnetization reducing pulse is applied after a gradient pulse is applied. The residual magnetization reducing pulse has a polarity opposite to the gradient pulse, and has an amplitude that can reduce residual magnetization caused by the gradient pulse. Thus, the residual magnetization after the application of the residual magnetization reducing pulse is reduced to a negligible degree. Therefore, the effect of residual magnetization caused by a gradient pulse can be sufficiently reduced.
In accordance with a second aspect of the invention, there is provided an MR imaging method comprising the steps of applying a killer pulse on a gradient axis, and thereafter applying a residual magnetization reducing pulse having a polarity and amplitude to reduce residual magnetization caused by the killer pulse.
In this configuration, the killer pulse is a gradient pulse for eliminating transverse magnetization by forcible dephasing. The killer pulse is also referred to as a spoiler pulse.
In the MR imaging method of the second aspect, a residual magnetization reducing pulse is applied after a killer pulse is applied. The residual magnetization reducing pulse has a polarity opposite to the killer pulse, and has an amplitude that can reduce residual magnetization caused by the killer pulse. Thus, the residual magnetization after the application of the residual magnetization reducing pulse is reduced to a negligible degree. Therefore, residual magnetization due to a killer pulse kp in an (ixe2x88x921)th FSE sequence SQ can be prevented from affecting data of an i-th FSE sequence SQ.
In accordance with a third aspect of the invention, there is provided an MR imaging method in which a phase encoding pulse is applied on a phase gradient axis, the method comprising the steps of: applying a residual magnetization reducing pulse after the phase encoding pulse, the residual magnetization reducing pulse having a polarity and amplitude to reduce residual magnetization caused by the phase encoding pulse, and additively increasing the area of the phase encoding pulse by the area of the residual magnetization reducing pulse.
In the MR imaging method of the third aspect, a residual magnetization reducing pulse is applied after a phase encoding pulse is applied. The residual magnetization reducing pulse has a polarity opposite to the phase encoding pulse, and has an amplitude that can reduce residual magnetization caused by the phase encoding pulse. Thus, the residual magnetization after the application of the residual magnetization reducing pulse is reduced to a negligible degree. Therefore, residual magnetization due to a phase encoding pulse can be prevented from affecting the subsequent data.
In accordance with a fourth aspect of the invention, there is provided an MR imaging method in which a phase encoding pulse is applied on a phase gradient axis and a rewinder pulse is applied after collecting an NMR signal, the method comprising the steps of: applying a residual magnetization reducing pulse after the phase encoding pulse, the residual magnetization reducing pulse having a polarity and amplitude to reduce residual magnetization caused by the phase encoding pulse, and additively increasing the area of the phase encoding pulse by the area of the residual magnetization reducing pulse, and further, applying a residual magnetization reducing pulse after the rewinder pulse, the residual magnetization reducing pulse having a polarity and amplitude to reduce residual magnetization caused by the rewinder pulse, and additively increasing the area of the rewinder pulse by the area of the residual magnetization reducing pulse.
In the MR imaging method of the fourth aspect, a residual magnetization reducing pulse is applied after a phase encoding pulse is applied. The residual magnetization reducing pulse has a polarity opposite to the phase encoding pulse, and has an amplitude that can reduce residual magnetization caused by the phase encoding pulse. Moreover, a residual magnetization reducing pulse is applied after a rewinder pulse is applied. The residual magnetization reducing pulse has a polarity opposite to the rewinder pulse, and has an amplitude that can reduce residual magnetization caused by the rewinder pulse. Thus, the residual magnetization after the application of the residual magnetization reducing pulse is reduced to a negligible degree. Therefore, residual magnetization due to a phase encoding pulse and a rewinder pulse can be prevented from affecting the subsequent data.
In accordance with a fifth aspect of the invention, there is provided an MR imaging method in which two or more gradient pulses having different polarities are successively applied on a gradient axis, comprising the step of determining respective amplitudes of the two or more gradient pulses to reduce residual magnetization after successively applying the two or more gradient pulses.
In the MR imaging method of the fifth aspect, because two or more gradient pulses having different polarities are successively applied, any new residual magnetization reducing pulse is not added, but instead, respective amplitudes of the two or more gradient pulses are adjusted to reduce residual magnetization to a negligible degree after successively applying the two or more gradient pulses. Therefore, residual magnetization after successively applying two or more gradient pulses can be prevented from affecting data.
In accordance with a sixth aspect of the invention, there is provided an MR imaging method in which a slice selective pulse is applied on a slice gradient axis and subsequently a rephasing pulse is applied, the method comprising the step of determining an amplitude of the rephasing pulse to reduce residual magnetization caused by the slice selective pulse.
In the MR imaging method of the sixth aspect, because a slice selective pulse and a rephasing pulse having different properties are successively applied, the amplitude of the rephasing pulse is adjusted to reduce residual magnetization to a negligible degree after successively applying the slice selective and rephasing pulses. Therefore, residual magnetization after successively applying a slice selective pulse and a rephasing pulse can be prevented from affecting data.
In accordance with a seventh aspect of the invention, there is provided an MR imaging method in which a dephasing pulse is applied on a read gradient axis and thereafter a read pulse is applied, comprising the step of determining an amplitude of the dephasing pulse so that the read pulse can reduce residual magnetization caused by the dephasing pulse.
In the MR imaging method of the seventh aspect, because a dephasing pulse and a read pulse having different polarities are successively applied, the amplitude of the dephasing pulse is adjusted to reduce residual magnetization to a negligible degree after successively applying the dephasing and read pulses. Therefore, residual magnetization after successively applying a dephasing pulse and a read pulse can be prevented from affecting data.
In accordance with an eighth aspect of the invention, there is provided an MR imaging method in which gradient moment nulling (GMN) phase compensation pulses are applied on a gradient axis, the method comprising the step of determining respective amplitudes of the GMN phase compensation pulses to reduce residual magnetization after applying the GMN phase compensation pulses.
In the above configuration, the GMN phase compensation pulse is defined as a gradient pulse having a waveform designed to eliminate motion-induced phase variation of nuclear spins.
In the MR imaging method of the eighth aspect, because GMN phase compensation pulses in which two or more gradient pulses having different polarities are combined are applied, the respective amplitudes of the gradient pulses are adjusted to reduce residual magnetization to a negligible degree after successively applying the GMN phase compensation pulses. Therefore, residual magnetization after successively applying GMN phase compensation pulses can be prevented from affecting data.
Thus, according to the MR imaging method and MRI apparatus of the present invention, for a gradient pulse having either a positive or negative polarity, a residual magnetization reducing pulse is applied after applying the gradient pulse, or for two or more successively applied gradient pulses having different polarities, the amplitude of the gradient pulse(s) is adjusted to reduce residual magnetization thereafter, and hence, an unwanted phase error does not occur and image quality degradation such as ghosting or shading can be prevented.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.